RFC3177, where the /48 recommendation was made, used the H ratio
analysis to explain why a /48 was acceptable. However the IETF did
not make any recommendation to the RIRs that the H ratio (current
version is now called HD ratio) should be used by the RIRs in their
allocation process nor what specific HD ratio be used. These choices
were made the RIRs when they developed their IPv6 allocation policies.
In my view the potential problems you describe have more to do with
the specific HD ratio the RIRs choose to use as opposed to the /48.
It's my understanding the HD-ratio value that the RIRs are now using
is much more conservative than before. This should, as I understand
it, avoid the overall problems you describe (e.g., cable modem, DLS,
Looking back at some old notes, I believe that changing the HD ratio
thresholds gave us roughly one order of magnitude's worth of
additional headroom. Moving the end site boundary from /48 to /56
gives us roughly two additional orders of magnitude savings. So, while
one can certainly argue about just how much is "enough" (since
projections 50-100 (or more!) years into the future are pretty iffy),
I think that both changes were important to make.
While I agree that 64K subnets is a lot of subnets for a home user,
my reluctance to formally change the /48 recommendation is that it
will lead to much more restrictive allocation policies. Just like
the one that started this thread:
IMO, this concern is overblown. And to be clear, I've heard this
concern for years -- especially here in the IETF -- and it was one of
the reasons why the 3177 recommendations were what they were. But
having attended RIR meetings on and off for the last 5 years, and
having talked with folk that go there, most of them very clearly
understand that we have lots of IPv6 addresses and that the intention
is that users get them. I do not hear serious talk about being
restrictive in giving out subnets.
What I do hear is that giving out /48 to everyone is simply
profligately wasteful. And unjustified. And repeats the early mistakes
of IPv4 where people didn't think about managing resources prudently.
We shouldn't be surprised that a "one size fits all" approach (where
home users get the same amount of space by default as an IBM or
Microsoft) doesn't seem to make a lot of sense to some people.
* /64 - Site needing only a single subnet.
* /60 - Site with 2-3 subnets initially.
* /56 - Site with 4-7 subnets initially.
* /52 - Site with 8-15 subnets initially.
* /48 - Site with 16+ subnets initially.
If this is what we would expect to get by changing the RFC3177 /48
recommendation, then in my view is not a good idea.
It should be noted that the policy proposal being quoted here was
submitted even though RFC3177 is on the books. And that some RIRs have
already adopted the change to /56. So, no, changing RFC 3177 isn't
(IMO) going to open the door (any wider) to such proposals.
At the same time the continuing existance of RFC 3177 is not going to
stop RIRs from adopting policies that they think make sense. Leaving
3177 on the books as it currently stands, strikes me as a form of
denial. It doesn't document existing practice, and at a minimum, we
should acknowledge that.
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